Based on the energy saving, space saving, non-polluting and the like requirements which have been called for in the printing ink, paint, coating, liquid resist ink and the like fields, extensive studies on photopolymerizable compositions and materialization efforts thereof have been made. The majority of these studies, however, have been based on the radical polymerization reaction of double bond. Though cationic polymerizable substances such as epoxy resins are excellent materials in terms of physical properties, it is difficult to effect their photopolymerization, so that materials in which double bonds are introduced by acrylic modification are mainly used.
In order to effect photo-curing of epoxy resins, a method has been proposed for example in U.S. Pat. No. 3,794,576 in which an epoxy resin is polymerized by using a photosensitive aromatic diazonium salt as a photopolymerization initiator and releasing a Lewis acid from the photopolymerization initiator by decomposing it with light irradiation. However, the aromatic diazonium salt releases not only Lewis acid but also nitrogen gas by its photo- decomposition, and the gas causes foaming of a paint film of the epoxy resin when its thickness becomes 15.mu. or more, so that the salt is not suited for use in thick coating. In addition, its mixture with epoxy resin causes a problem in terms of storage stability, such as gradual progress of hardening even in the absence of light, so that it is difficult to make the mixture into a one-part composition. Thus, various attempts have been made with the aim of overcoming disadvantages of the aforementioned diazonium salt-based initiators, and, as techniques for the improvement of thick coating property and storage stability, aromatic sulfonium salt-based and aromatic iodonium salt-based initiators and curable resin compositions containing the same have been disclosed for example in Japanese Patent Publication No. 52-14278(1977), Japanese Patent Publication No. 52-14277(1977), Japanese Patent Application Kokai No. 54-53181(1979), Japanese Patent Publication No. 59-19581(1984) and Japanese Patent Application Kokai No. 56-55420(1981). However, these colored compositions are poor in curability, and its improving means have not been proposed. In addition, with the expansion of fields in which photopolymerizable compositions are used, it becomes important to provide a novel composition in order to meet the needs of the market.